For purposes of diagnosis and treatment planning, imaging techniques are commonly used in medical procedures to view the internal anatomy of a patient's body. In one imaging technique, an ultrasound device with one or more ultrasound transducers mounted on its tip is inserted into the patient's body, e.g., through a blood vessel. To obtain an interior image of the body, the ultrasound transducer emits pulses of ultrasound energy into the body. A portion of the ultrasound energy is reflected off of the internal anatomy of the body back to the transducer. The reflected ultrasound energy (echo) impinging on the transducer produces an electrical signal, which is used to form the interior image of the body. To provide a planar or sector view of the surrounding tissue, the ultrasound device will typically have either one or more rotating transducers or a phased array of transducers that are mechanically disposed about the circumference or along the axis of the ultrasound device.
In order to assist physicians in maneuvering medical devices (e.g., imaging devices) to sites of interest in the body, several guidance systems have been developed. In one guidance system, a fluoroscopic image of the device (or at least the radiopaque bands of the device) and surrounding anatomical landmarks (with or without the use of contrast media) in the body are taken and displayed to the physician. The fluoroscopic image enables the physician to ascertain the position of the device within the body and maneuver the device to the site of interest. In another guidance system using anatomic mapping, a graphical representation of the device or portion of the device is displayed in a 3-D computer-generated representation of a body tissue, e.g., heart chamber. The 3-D representation of the body tissue is produced by mapping the geometry and/or electrical activity of the inner surface of the body tissue in a 3-D coordinate system by moving a mapping device to multiple points on the body tissue. The position of the device to be guided within the body tissue is determined by placing one or more location sensors on the device and tracking the position of these sensors within the 3-D coordinate system. An example of this type of guidance system is the Realtime Position Management™ (RPM) tracking system developed commercially by Cardiac Pathways Corporation, now part of Boston Scientific Corp. The RPM system is currently used in the treatment of cardiac arrhythmia to define cardiac anatomy, map cardiac electrical activity, and guide an ablation catheter to a treatment site in a patient's heart.
Although the utility of present guidance techniques for guiding devices to sites of interest in the body has been proven, they are limited in their ability to localize the specific body tissue that is being imaged by imaging devices at any given instant. As a result, it is difficult for physicians to determine what portion of the body he or she is imaging with imaging devices or to determine the location of those imaging devices relative to the patient's anatomy using present localization techniques.